The present invention relates to a method for producing a tetrafluoroethylene/perfluoro(alkyl vinyl ether) type copolymer excellent in stability.
A tetrafluoroethylene (hereinafter referred to as TFE)/perfluoro(alkyl vinyl ether) (hereinafter referred to as PAVE) type copolymer (hereinafter referred to as PFA) is known as a melt processable fluororesin and is widely used as a material for formed products such as tubes, pipes, joints and containers, for wire coatings, for coating, for lining, etc.
When PFA is used for containers, pipings or joints for a liquid reagent for semiconductors, there has been a problem that fluorine ions tend to elute from PFA, and cracking is likely to form by the liquid reagent. In such an application, it is common to use PFA stabilized by treatment with fluorine gas (hereinafter referred to fluorination) to convert unstable terminal groups in the molecule to stable terminal groups in order to reduce elution of fluorine ions or to improve the cracking resistance against a liquid reagent. PFA stabilized by fluorination is free from a problem such that the unstable terminal groups are decomposed during injection molding to form hydrofluoric acid which in turn corrodes a mold and has merits for molding, as compared with conventional PFA having unstable terminal groups.
However, for the production of PFA stabilized by fluorination, a treatment installation employing a fluorine gas is required, and there has been a problem that the production process tends to be cumbersome.
As the terminal groups in PFA, the following terminal groups are conceivable.
When PFA is produced by solution polymerization or suspension polymerization, terminal groups derived from a polymerization initiator tend to be substantial. In the production of PFA, as the polymerization initiator, a fluorine type polymerization initiator of e.g. (X(CF2)nCOO)2 (wherein X is a hydrogen atom, a fluorine atom or a chlorine atom, and n is an integer of from 1 to 10) is preferred. As a result, the terminal groups derived from the polymerization initiator will be stable terminal groups such as X(CF2)n groups.
On the other hand, the following three types are conceivable as unstable terminal groups in PFA which cause elution of fluorine ions or cracking by a liquid reagent.
(1) Terminal groups derived from a chain transfer agent.
(2) xe2x80x94COF terminal groups formed by a transition reaction of radicals formed when PAVE is added to growing chain radicals in the copolymerization process of TFE and PAVE as shown by the formula 1 and disclosed in JP-B-4-83: 
wherein Rf is a perfluoroalkyl group.
(3) xe2x80x94COOH terminal groups formed by hydrolysis of xe2x80x94COF groups.
Among them, the number of unstable terminal groups derived from a chain transfer agent is large. Such terminal groups derived from a chain transfer agent are different depending upon the type of the chain transfer agent to be employed.
U.S. Pat. No. 3,642,742 discloses a case where cyclohexane and methanol are used as chain transfer agents. With cyclohexane, cyclohexyl groups will be formed as terminal groups, and with methanol, xe2x80x94CH2OH groups are formed. When either one of the chain transfer agents was used, the thermal stability of the formed PFA was low.
When methane is used as a chain transfer agent, xe2x80x94CH3 groups will be formed as terminal groups. As compared with xe2x80x94CH2OH groups, xe2x80x94COF groups and xe2x80x94COOH groups, the stability of PFA will be improved with xe2x80x94CH3 groups. However, even then, the stability is not sufficient for an application to semiconductors, and it has been necessary to change xe2x80x94CH3 groups to xe2x80x94CF3 groups by fluorination for the purpose of stabilization.
Accordingly, it has been desired to develop a method for producing stable PFA, which does not require a treatment process employing a fluorine gas.
It is an object of the present invention to provide a method for producing PFA having a small content of unstable terminal groups contained in the molecule.
The present invention provides a method for producing PFA which comprises carrying out polymerization in a polymerization medium in the presence of a chain transfer agent by means of a polymerization initiator, wherein the chain transfer agent is a C1 or 2 hydrofluorocarbon.
In the method for producing PFA according to the present invention, as the polymerization method, a known polymerization method such as suspension polymerization, solution polymerization, emulsion polymerization or bulk polymerization may be employed without any particular limitation. Particularly preferred is solution polymerization or suspension polymerization.
In the method for producing PFA according to the present invention, TFE and PAVE which are monomers, are co-polymerized.
As PAVE, a monomer represented by CF2xe2x95x90CFORf is employed. Rf is a perfluoroalkyl group, preferably a C1-6 linear, branched or cyclic perfluoroalkyl group, more preferably a linear perfluoroalkyl group, still further preferably, a perfluoromethyl group, a perfluoroethyl group or a perfluoro n-propyl group.
In the present invention, in addition to TFE and PAVE, other monomers may be copolymerized in a small amount. As such other monomers, fluorine-containing olefins other than TFE, such as trichlorofluoroethylene and hexafluoropropylene, may be mentioned.
In the present invention, the ratio of polymerized units of TFE to polymerized units of PAVE in PFA is preferably such that polymerized units of TFE/polymerized units of PAVE are equal to from 99.5/0.5 to 95/5 (molar ratio). If the ratio is smaller than this range, the processability and stress cracking resistance tend to deteriorate, and if it is larger than this range, the mechanical properties tend to deteriorate. More preferably, polymerized units of TFE/polymerized units of PAVE are equal to from 99/1 to 97/3 (molar ratio).
The content of polymerized units of other monomers is preferably from 0 to 10 mol %, more preferably from 1 to 8 mol %, based on the total mol amount of polymerized units of TFE and polymerized units of PAVE.
As the polymerization medium in the present invention, a fluorine-containing organic medium having a small chain transfer coefficient is preferred. Particularly preferred is at least one polymerization medium selected from the group consisting of a C3-10 perfluorocarbon, a C3-10 hydrofluorocarbon and a C3-10 hydrochlorofluorocarbon.
The perfluorocarbon is preferably a saturated perfluorocarbon having a linear, branched or cyclic structure (provided that it may contain an etheric oxygen atom in its molecule). Specific examples include perfluorocyclobutane, perfluorohexane, perfluoro(dipropyl ether), perfluorocyclohexane and perfluoro(2-butyltetrahydrofuran).
The hydrofluorocarbon is preferably a saturated hydrofluorocarbon having a linear, branched or cyclic structure, wherein the number of fluorine atoms in the molecule is larger than the number of hydrogen atoms (provided that it may contain an etheric oxygen atom in the molecule). Specific examples include CH3OC2F5, CH3OC3F7, C5F10H2, C6F13H and C6F12H2.
The hydrochlorofluorocarbon is preferably a hydrochlorofluorocarbon having a linear, branched or cyclic structure, wherein the number of hydrogen atoms is at most 3 (provided that it may contain an etheric oxygen atom in the molecule). Specific examples include CHClFCF2CF2Cl.
The amount of the polymerization medium to be used is preferably from 10 to 90%, more preferably from 30 to 70%, by volume %, based on the volume of the autoclave.
As the chain transfer agent in the present invention, a C1 or 2 hydrofluorocarbon is used. Further, a hydrofluorocarbon wherein the molar ratio of number of hydrogen atoms/number of fluorine atoms in its molecule is from 1/2 to 5/1, is preferred. Specific examples include CF2H2, CFH3, CFH2CH3, CF2HCH3, CFH2CFH2, CF2HCFH2, CF3CH3 and CF3CFH2. Further, from the viewpoint of the chain transfer property, a hydrofluorocarbon having a larger number of hydrogen atoms than the number of fluorine atoms in its molecule, is preferred. By the use of such a chain transfer agent, PFA excellent in stability will be formed.
The amount of the chain transfer agent is preferably from 0.1 to 50 mass %, more preferably from 1 to 40 mass %, based on the amount o the polymerization medium.
As the polymerization initiator in the present invention, a fluorocarbon type diacyl peroxide is preferred since it forms PFA having stable terminal groups. Particularly preferred is a diacyl peroxide represented by (X(CF2)nCOO)2 (wherein X is a hydrogen atom, a fluorine atom or a chlorine atom, and n is an integer of from 1 to 10). By the use of such a polymerization initiator, PFA excellent in the stability will be formed.
Specific examples of such a diacyl peroxide include (CF3COO)2, (CF3CF2COO)2, (CF3CF2CF2COO)2, HCF2CF2COO)2, (HCF2CF2CF2COO)2, (ClCF2CF2COO)2 and (ClCF2CF2CF2COO)2.
The amount of the polymerization initiator is preferably from 0.01 to 1%, more preferably from 0.01 to 0.5%, by mass %, based on TFE to be post charged.
The polymerization temperature in the present invention is preferably from 20 to 75xc2x0 C. If the temperature is higher than this range, the number of xe2x80x94COF terminal groups to be formed, tends to increase, and if it is lower than this range, the decomposition speed of the initiator tends to be slow. More preferably, it is from 30 to 60xc2x0 C.
The polymerization pressure in the present invention is preferably from 0.7 to 10 MPa. If the polymerization pressure is too low, the content of xe2x80x94COF terminal groups in PFA increases, and if the polymerization pressure is too high, such is not desirable for the production equipment. It is more preferably from 0.8 to 5 MPa and most preferably from 1 to 3 MPa.
PFA to be produced by the method of the present invention preferably has an apparent melt viscosity of from 1xc3x97102 to 1xc3x97105 Paxc2x7s at 380xc2x0 C. If the apparent melt viscosity is too low, the processability tends to be low, thus leading to surface roughening, and if the apparent melt viscosity is too high, the mechanical properties and heat resistance tend to deteriorate. More preferably, it is from 1xc3x97103 to 5xc3x97104 Paxc2x7s.
PFA produced by the method of the present invention, may be processed by a method such as injection molding, extrusion, transfer molding, rotational molding or electrostatic powder coating.
PFA produced by the method of the present invention is excellent in the applicability to tubes, hoses, containers, joints, etc., to be used particularly in connection with the equipment for the production of semiconductors. Further, other than in connection with the equipments for the production of semiconductors, it is excellent also in the applicability to components, pipings or tank linings of chemical plant installations, or to injection molded components, copy rolls, etc. of e.g. office automation equipments.